Semiconductor image sensors are used for converting optical images into electrical signals and they may be categorized into complementary metal-oxide-semiconductor (CMOS) image sensors and charge-coupled device (CCD) sensors, which are widely used in various applications such as digital still camera or mobile phone camera applications.
Generally, CMOS image sensors are gaining in popularity over traditional charged-coupled devices due to certain advantages inherent in the CMOS image sensors. Particularly, CMOS image sensors typically require lower voltages, consume less power, enable random access to image data, may be fabricated with compatible CMOS processes, and enable integrated single-chip cameras. In general, CMOS image sensors utilize light-sensitive CMOS circuitry, which may include a photosensitive element in a silicon substrate, to convert light energy into electrical energy. As the photosensitive element is exposed to light, an electrical charge is induced in the photosensitive element. The charge generated by the photosensitive element is then controlled and sampled by a MOS switching transistor.
CMOS image sensors can be further divided into two major categories, namely front-side illuminated (FSI) image sensors and back-side illuminated (BSI) image sensors depending on the light path difference. In a FSI image sensor, light from the subject scene is incident on the front side of the CMOS image sensor, passes through dielectric layers and interconnect layers, and finally falls on the photo diode. The additional layers (e.g., opaque and reflective metal layers) in the light path may limit the amount of light absorbed by the photosensitive element so as to reduce quantum efficiency. In contrast, there is no obstruction from additional layers (e.g., metal layers) in a BSI image sensor. Light is incident on the backside of the CMOS image sensor. As a result, light can strike the photosensitive element through a direct path. Such a direct path helps to increase the number of photons converted into electrons.